Document the deprecation of WFlags

Task-number: QTBUG-31737
Change-Id: I95dab7b174438b27e6fd555f88bca78099656d0c
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>